A typical method for producing a wafer for a substrate in electronics industry includes 1) a slicing step of slicing a single crystal ingot to obtain a thin circle wafer, 2) a chamfering step of chamfering a circumferential part of the wafer, 3) a lapping step of planarizing the chamfered wafer, 4) an etching step of removing processing strain of the lapped wafer, 5) a polishing step of forming a mirror surface of the etched wafer and, 6) a washing step of washing the polished wafer.
The polishing step is carried out by pressing the wafer as a target to be polished against a polishing pad and relatively moving the wafer while a polishing liquid composition is supplied onto the surface of the polishing pad. The polishing step generally includes a plurality of stages of a first polishing, a second polishing, and a final polishing. The first polishing and the second polishing are carried out for the purpose of removing deep scratches on the wafer surface generated in the lapping step and the etching step.
On the other hand, the final polishing is carried out for the purpose of removing fine surface defects remaining after the first polishing and the second polishing and planarizing the surface in high accuracy. As evaluation criteria of the quality of the wafer after the final polishing, a light point defect (LPD) and a haze (a degree of surface clouding) are generally used.
A LPD means fine surface defects that cause diffuse reflection when a wafer having a mirror surface is irradiated with strong light. The fine surface defects result from scratches caused by coarse abrasive grains or foreign substances during polishing, attached substances such as abrasive grains or foreign substances, or a deteriorated processed layer caused by attachment of abrasive grains, foreign substances, or the like.
On the other hand, the haze means a degree of clouding generated by diffuse reflection that is caused by sub-nano level roughness of the wafer surface when a wafer having a mirror surface is irradiated with strong light. As the flatness of the wafer increases, the haze becomes better. It can be said that the smaller the number of LPDs and a value of the haze are, the better the quality of the wafer is.
In the final polishing process carried out for the purpose of improving the LPD and the haze, a polishing liquid composition is generally used in which an alkaline compound is added and then a water-soluble polymer compound is added to silica particles dispersed in water. The water-soluble polymer compound having a stress relaxation function is effective not only in reducing damage caused by abrasive grains or foreign substances but also in providing the hydrophilicity to the wafer surface and preventing attachment of abrasive grains or foreign substances. By adding a compound having an alcoholic hydroxy group, which improves the hydrophilicity of the wafer surface, the effects of reducing scratches and preventing the attachment are more improved and thus highly accurate planarization can be achieved.
Cellulose derivatives, as represented by hydroxyethyl cellulose, which have been conventionally used as water-soluble polymers, tend to cause fluctuation of quality because the cellulose derivatives are made of natural cellulose as a raw material. As a result, the LPD is often worsened by the use of the cellulose derivatives. On the other hand, although synthetic water-soluble polymers as represented by a polyvinyl alcohol have no fluctuation of quality, improvement of the haze is required.
Patent Document 1 describes a final polishing composition containing an ethylene oxide-modified polyvinyl alcohol. However, the haze is confirmed by visual observation and thus a degree of improvement is uncertain. Patent Document 2 describes a polishing composition containing an anion-modified polyvinyl alcohol. However, improvement of the haze is not described clearly.